Heterocycle/Heteroallene Ring‐Opening Copolymerization: Selective Catalysis Delivering Alternating Copolymers

Abstract Heteroatom‐containing polymers have strong potential as sustainable replacements for petrochemicals, show controllable monomer–polymer equilibria and properties spanning plastics, elastomers, fibres, resins, foams, coatings, adhesives, and self‐assembled nanostructures. Their current and future applications span packaging, house‐hold goods, clothing, automotive components, electronics, optical materials, sensors, and medical products. An interesting route to these polymers is the catalysed ring‐opening copolymerisation (ROCOP) of heterocycles and heteroallenes. It is a living polymerization, occurs with high atom economy, and creates precise, new polymer structures inaccessible by traditional methods. In the last decade there has been a renaissance in research and increasing examples of commercial products made using ROCOP. It is better known in the production of polycarbonates and polyesters, but is also a powerful route to make N‐, S‐, and other heteroatom‐containing polymers, including polyamides, polycarbamates, and polythioesters. This Review presents an overview of the different catalysts, monomer combinations, and polymer classes that can be accessed by heterocycle/heteroallene ROCOP.


Introduction
Synthetic polymers are structurally tuneable materials integral to modern life,whether for commodity products,such as clothing, food packaging,o rh ouse-hold goods,o rf or specialist applications,s uch as microelectronics,r enewable energy generation, or robotics. [1] With worldwide production volumes exceeding 370 Mt onnes annually,apolymer-free society is at best avague memory rather than avision for the future. [2] Thes uccess story of polymers from last century has its origin in their close coupling with the liquid fuel industry, optimized production methods,l ow costs,a nd immense chemical diversity;t hese features allow material properties to be precisely tailored to ah uge range of different applications.I nt he coming century,amove away from petrochemical raw materials allows am ore widespread consideration of chemistry beyond hydrocarbons.T here should be advantages to such chemical approaches which are closer to monomer-polymer equilibria and hence should facilitate complete depolymerisation, chemical recycling, and even bio-degradation. [2][3][4] Heteroatom-containing polymers are experiencing ar enaissance,b oth because of their potential to address such sustainability priorities as well as to deliver new or better properties that target future applications.C urrently,sulfur cross-linked polymers are essential as rubbers and engineering thermoplastics,while amide linkages are,ofcourse,integral to the performances of nylon fibres and urethanes as well as PU foams. [1,[5][6][7] Most of these materials are made by versatile and scalable polycondensations.Future economic,e nvironment, and technological challenges may benefit from materials better tailored to the application.
One option is to improve structural control and hence provide insight into structure-property relationships of features such as heteroatom placement, functionalised block Heteroatom-containing polymers have strong potential as sustainable replacements for petrochemicals,s howcontrollable monomer-polymer equilibria and properties spanning plastics,elastomers,f ibres, resins,f oams,coatings,a dhesives,a nd self-assembled nanostructures. Their current and future applications span packaging, house-hold goods,clothing, automotive components,electronics,optical materials, sensors,and medical products.Aninteresting route to these polymers is the catalysed ring-opening copolymerisation (ROCOP) of heterocycles and heteroallenes.Itisaliving polymerization, occurs with high atom economy,a nd creates precise,n ew polymer structures inaccessible by traditional methods.I nt he last decade there has been arenaissance in researcha nd increasing examples of commercial products made using ROCOP.Itisbetter known in the production of polycarbonates and polyesters,but is also apowerful route to make N-, S-, and other heteroatom-containing polymers,i ncluding polyamides, polycarbamates,and polythioesters.This Review presents an overview of the different catalysts,monomer combinations,and polymer classes that can be accessed by heterocycle/heteroallene ROCOP. polymer sequence,a rchitecture,m olar mass value,a nd distribution as well as regio-and stereochemistry.Delivering such tunability requires controlled polymerisation methods and one interesting and generally applicable option to install heteroatoms is heterocycle/heteroallene ring-opening copolymerisation (ROCOP;F igure 1). [8,9] Controlled, or living, polymerization shows complete and fast chain initiation followed by uniform propagation rates,limited side reactions, and triggered termination. Ther esulting polymers show predictable molar mass values,n arrow dispersities,c ompositions dictated by the starting monomer stoichiometry,a nd high end-group fidelity;a st he growing polymer chain ends are "living" they may be used to make (block) copolymers and more complex architectures with unparalleled selectivity. [10][11][12] Heterocycle/heteroallene ROCOP typically affords functional polymers such as (thio)esters,( thio)carbonates, carbamates,orurethanes. Heterocycle/heteroallene ROCOP dates from the 1960s and renewed attention is partly driven by the potential for sustainability,asmonomers such as CO 2 ,COS,SO 2 ,and S 8 are industrial wastes and others may be bio-derived. [13,14] Appli-cations of these materials naturally depend upon their polymer chemistry and physics,b ut low molar mass ROCOP polyols are already showing promise as surfactants, coatings,adhesives,orfoams,and higher molar mass polymers are showing promise as high refractive index materials, absorbents,supports,a nd high-performance plastics. [15] This Review presents the principles of heterocycle/heteroallene ROCOP catalysis as at ool for polymer synthesis.I t introduces the polymerisation methodology,u sing wellknown monomer combinations such as carbon dioxide/ epoxide and anhydride/epoxide ROCOP,a nd then presents other rarer monomer combinations,w ith as pecial focus on polymers containing O, N, or Sh eteroatoms ( Figure 2). Research is progressing fast using such specialized monomer combinations but reproducible,e ffective,a nd selective polymer syntheses are essential prior to optimizations of the material properties.Insome cases,the activity and selectivity of the ROCOP catalysis is rather low and here areas for future development are highlighted that are driven by promising initial polymer property data. TheR eview is focussed on polymerisation catalysis,w here advances will allow future explorations of polymer properties,p rocessing, and applications.

Recent Trends in CO 2 /Epoxide ROCOP
Arguably the most widely investigated ROCOP is that of CO 2 and epoxides.O ver the last 50 years this reaction has advanced from al aboratory curiosity to ac ommercialised technology. [16,17] This monomer combination serves as an exemplar of the conceptual advances both in catalysis and product performances.T here are already several excellent reviews on CO 2 /epoxide ROCOP and the interested reader is directed to these other reports for comprehensive coverage of this field. [15,[18][19][20][21][22] Thec haracteristics of CO 2 /epoxide ROCOP provide excellent illustrations of principles that are generally applicable to other, more unusual monomer combinations, and key developments can also inform about future research directions for other polymers.
Firstly,t he elementary steps of CO 2 /epoxide ROCOP ( Figure 3) are described, since most are general to other heteroallene/heterocycle combinations: 1) Initiation occurs when ac oordinated epoxide is ringopened, by the initiator/catalyst, to form an alkoxide, which becomes the propagating species.T he initiator can be ametal-ligand complex, an ionic co-catalyst, or aLewis base. 2) Propagation occurs by two alternating processes:a )CO 2 insertion, where the alkoxide transforms into acarbonate intermediate.M any catalysts show CO 2 insertion kinetics that are considerably faster than epoxide ring-opening. b) Epoxide ring opening,w here the carbonate intermediate ring opens the epoxide to regenerate the alkoxide.F or many catalysts this is the "rate-determining step". 3) Te rmination occurs when the catalyst is permanently deactivated, usually by irreversible protonolysis achieved by adding excess water, acid, or even atmospheric moisture into the polymerisation.
Other processes can also occur and these may or may not be desired: 1) Backbiting occurs when the alkoxide or carbonate intermediates attack the polymer chain rather than an ew monomer. This process generates "cyclic five-membered carbonate" (c5c) instead of progressing chain growth. Form ost carbon dioxide/epoxide coupling reactions,c 5c is the thermodynamic product and polycarbonate is the kinetic product.
2) Epoxide homopropagation occurs when the alkoxide intermediate reacts with as econd epoxide molecule, rather than with carbon dioxide.Itresults in the formation of ether,oreven polyether, linkages in the polymer chain. 3) Chain transfer occurs when deliberately added protic compounds,t ypically alcohols,w ater, or carboxylic acids, undergo ar apid and reversible series of exchange reactions with the propagating alkoxide or carbonate intermediates. [23] Catalysts able to undergo controlled chain-transfer reactions can be very useful, for example, for precise control over the polymer molar mass or endgroup functionality.T hese reactions are also exploited to produce multi-functional star or branched materials.I n some contexts,aform of chain transfer is applied to "start" the polymerisations.I no ther contexts,c hain transfer occurs from 1,2-diols formed by epoxide hydrolysis,which is attributed to low levels of water contamination. [23,24] In such cases,t he resulting polycarbonates show bimodal molar mass distributions,a saresult of chains initiated both by the catalyst and diol. [25,26] Some catalysts are very tolerant of chain-transfer agents,t hereby delivering precisely controlled molar mass and directing specific applications. [27,28] Low molar mass,h ydroxy-end-capped polycarbonates are useful as surfactants,p olyols,o rr esin components and for making higher polymers,particularly polyurethanes. [7] Cross-linking reactions either using endor side-chain substituents deliver coatings,r esins,o r thermosets.H igher molar mass polycarbonates are used as elastomers,films,and rigid plastics. [15] Heterocycle/heteroallene ROCOP is critically dependent upon the catalyst selection, and we provide here an outline of the best performing systems.One widely investigated class are metal(III)-salen or porphyrin catalyst systems which comprise the use of Cr III ,C o III ,o rA l III complexes together with anucleophilic co-catalyst, often asoluble "onium"-halide salt (consisting of aw eakly coordinating cation such as R 4 N + ,  refers to ametal catalyst, "P" to the polymer chain, Xt he initiating coligand. R 4 P + ,orPh 3 P=N + =PPh 3 (PPN) and, for example,Cl À or Br À ) or an organic base (e.g.d iazabicycloundecene (DBU) or triazabicyclodecene (TBD)). [20,[29][30][31][32] These catalyst systems polymerize through bimetallic and/or monometallic mechanisms ( Figure 4). Often both mechanisms appear to occur in parallel, differing by epoxide activation at the same or ad ifferent metal to the site of carbonate coordination. In either case,c atalytic activity is lost or severely compromised without the co-catalyst and this means that activity also falls rapidly below ac ritical ion-pair concentration. [29] At high temperatures,such systems typically deliver large amounts of c5c. [20] To address these difficulties,m any catalysts were redesigned to attach the co-catalyst to the ancillary ligand;this strategy led to outstanding activity values,a mongst the highest reported in this field ( Figure 5). Nozaki and coworkers pioneered this strategy,r eporting ap iperidiniumtethered Co III -salen complex that showed excellent activity for CO 2 /propene oxide (PO) ROCOP. [33] Later,L ee and coworkers reported aC o III -salen complex featuring two tethered silylammonium dinitrophenolate co-catalysts which showed remarkable activity at al ow catalyst loading. [34] Even higher rates were achieved by tethering four ammonium(dinitro)phenolate substituents to the Co III -salen complex, although it should be noted the catalyst syntheses were lengthy. [35] Recently,N ozaki and co-workers reported af our-arm tethered Al III -porphyrin catalyst that showed excellent activity for CO 2 /cyclohexene oxide (CHO) ROCOP (TOF 10 000 h À1 ,120 8 8C, 0.0025 mol %, 99 %PCHC). [37] These catalyst-tethered systems show af irst order dependence on the overall catalyst concentration, whereas the analogous bicomponent system shows af ractional order in the metal complex. [36] Another advantage is the improved selectivity for polymer,w hich is attributed to reduced backbiting from uncoordinated chains by electrostatic attractions "holding" any free anions "close" to the cationic catalyst. Neutral cocatalysts were also tethered to metal complexes to deliver rate enhancements,f or example,L ua nd co-workers applied aT BD-tethered Co III -salen complex that was six times more active than the bicomponent equivalent and maintained activity at low dilution (0.01 mol %). [38] Organocatalysts (although often featuring metals from Groups 1-13) are also active for CO 2 /epoxide ROCOP and are typically bicomponent systems comprising aL ewis acid/ base pair. Some of these organocatalysts may be attractive in terms of ease of use on asmall scale and their lack of colour. Feng and co-workers discovered that Et 3 B( Lewis acid) and PPNCl (Lewis base) combinations showed good activity for both CO 2 /PO (TOF 49 h À1 ,6 0 8 8C, 0.1 mol %, 10 bar, 83 % PPC) and CO 2 /CHO ROCOP (TOF 600 h À1 ,8 0 8 8C, 0.025 mol %, 10 bar,9 9% PCHC). [39,41,42] DFT investigations support the rate-determining step involving triethylborane epoxide and coordination of the propagating chain end ( Figure 6);t his mechanism closely resembles earlier reports for metal/co-catalyst bimetallic pathways.V ery recently,W u and co-workers reported aquaternary ammonium-tethered 9borabicyclo(3.3.1)nonane,a pplied at 0.005 mol %l oading, which shows high activity for CO 2 /CHO ROCOP (TOF 4900 h À1 ,1 50 8 8C, 15 bar). [40] Ar elated ammonium salt quadruply tethered to borane moieties achieves CO 2 /epichlorhydrin (ECH) ROCOP to produce awhite polymer (TOF 7h À1 , 25 8 8C, 0.1 mol %, 25 bar,9 9% polymer;F igure 7). [43] Reports proposing bimetallic mechanisms for bicomponent systems motivated the preparation of di-and multimetallic catalysts,m any of which operate without ac ocatalyst. [18] In the best cases,t hese catalysts are as active as tethered catalyst/co-catalyst systems and may be simpler to make and apply,s ince they maintain high activity at am uch low CO 2 pressure. [43] Coates et al. pioneered Zn II -diketimide catalysts ( Figure 8) and through elegant kinetic investigations established the most active were loosely associated dimers;t his excellent work has already been thoroughly reviewed. [21] Lee et al. reported Zn II 2 -bis(anilido-aldimide) catalysts for CO 2 / CHO ROCOP (TOF 312 h À1 ,808 8C, 0.02 mol %, 12 bar,94% Figure 5. Metal-salen catalysts with atethered co-catalyst;X= 2,4dinitrophenolate. [33][34][35][36] Figure 6. Proposed mechanism for bicomponent organocatalysed CO 2 /epoxide ROCOP. [39,40]  PCHC) and noted that electron-withdrawing substituents on the ligand enhanced the activity but reduced the selectivity (TOF 2860 h À1 ,808 8C, 0.002 mol %, 14 bar,79% PCHC). [44,46] Later, Rieger and co-workers reported ahighly active macrocyclic Zn II 2 -bis(diketimide) catalyst (TOF 9130 h À1 , 100 8 8C, 0.025 mol %, 40 bar, > 99 %P CHC). [45] Curiously,i tf ollows somewhat complex kinetics that are dependent on the carbon dioxide pressure,w ith az ero order at 5-25 bar changing to first order at 25-45 bar.F urther studies showed that more rigid macrocycles reduced the activity, [47] whereas electronwithdrawing substituents increased it (TOF 155 000 h À1 100 8 8C, 0.0125 mol %, 30 bar,8 8% PCHC). [48] Since 2008, our group has investigated catalysts featuring metal coordination to macrocyclic diphenolate tetraamine ligands. [49] Thef irst report described Zn II 2 catalysts which showed moderate activity at 1bar CO 2 (TOF 18 h À1 ,8 08 8C, 0.1 mol %) and were,a tt hat time,arare example of lowpressure catalysts.S ubsequently, Mg 2 (TOF 35 h À1 ), Co II 2 (TOF 161 h À1 ), and Fe III 2 (TOF 6h À1 ) catalysts have all showed activity at 1bar CO 2 (80 8 8C, 0.1 mol %). [50][51][52] Detailed investigations of the polymerisation kinetics,D FT calculations, in situ spectroscopy,a nd structure-activity studies supported ac hain shuttling mechanism in which the polymer chain moves between the two metal centres with each monomer insertion. [53] Ak ey outcome from this mechanism was the potential for heterodinuclear catalysts,since each metal was attributed ad istinct role in the catalytic cycle ( Figure 9). Heterodinuclear Zn II Mg II catalysts showed greater activity than either homodinuclear analogue, that is,Z n II Zn II or Mg II Mg II . [58,59] This work provided the first evidence of catalytic synergy and supported the hypothesis that each metal has ad istinct mechanistic function. Va riants of this Zn II Mg II catalyst, featuring organometallic, non-initiating C 6 F 5 coligands,a nd applied with alcohol as ac hain-transfer agent, resulted in both high activity and selectivity for telechelic polycarbonates. [60] Theorganometallic Zn II Mg II catalysts react with the diols to form the desired alkoxide initiators in situ and deliver accurate control over the molar mass and end-group chemistry.T hese catalysts were used to prepare polycarbonate-b-polyester-b-polycarbonate ABAtriblock copolymers from mixtures of CHO,CO 2 ,and bio-based e-decalactone.T he catalysts deliver precise control of the bock ratios and carbon dioxide contents (6-23 wt %). By controlling the carbonate linkage content, the material properties of the polymers were tuned from adhesives to elastomers to ductile plastics,thus addressing in particular the brittleness of the parent PCHC segments.I n2 018, Mashima and co-workers reported aseries of high activity multimetallic catalysts featuring Zn II 3 Ln(III) for CO 2 /CHO ROCOP (TOF 300 h À1 ,1 00 8 8C, 0.05 mol %, 3bar CO 2 ). In 2020 the same ligand was used to produce more active Co II 3 Ln(III) catalysts (TOF 1625 h À1 ,0 .004 mol %, 20 bar). [56,61] In 2020, our group investigated the phenomena which underpin the catalytic synergy in CHO/CO 2 ROCOP by using Figure 7. CO 2 /ECH copolymerisation by an ammonium-borane catalyst produces awhite polycarbonate. [43] Copyright 2021 AmericanC hemical Society. Figure 8. Bimetallic active site for CO 2 /CHO ROCOP proposed for zinc diketimide catalysts. [21,44,45] Figure 9. Heterodi-and multinuclear catalysts for CO 2 /epoxide ROCOP. [53][54][55][56][57] ah eterodinuclear Mg II Co II catalyst ( Figure 10). [54] This catalyst showed an excellent activity at 1bar (TOF 1205 h À1 , 120 8 8C, 0.05 mol %, 99 %P CHC) and 20 bar CO 2 pressure (TOF 12 460 h À1 ,140 8 8C, 0.05 mol %, 99 %PCHC). It showed ar ate four times higher than Mg II Mg II and double that of Co II Co II catalysts.D etailed kinetic analyses showed that j DS°j is reduced for Mg II -containing catalysts,w hilst DH°is smaller for Co II -containing variants.Hence,the success of the heterodinuclear catalyst is attributed to Mg II coordinating the epoxide with ar educed transition-state entropy,w hile the Co II -carbonate attacks it with ar educed transition-state enthalpy.S ynergy arises because each metal has ad istinct role in the catalytic rate-determining step and the kinetics provide experimental evidence for this proposition. Epoxide ring opening transfers the propagating alkoxide to the Mg II site and carbon dioxide insertion results in the chain "shuttling" back to the Co II centre ready for the next cycle of monomer insertions.
In 2020, we also reported ah eterodinuclear Co III K catalyst, coordinated by an asymmetric diphenolate,diamine macrocycle featuring at etra-ether moiety,w hich showed excellent activity in PO/CO 2 ROCOP (TOF 800 h À1 ,7 08 8C, 0.025 mol %, 30 bar CO 2 ,93% PPC). [62] Notably this catalyst tolerates up to 250 equivalents of chain transfer agent, and is thus useful for the production of polycarbonate polyols. Another heterotrimetallic catalyst featuring Zn 2 Na also shows good activity for CO 2 /CHO ROCOP at 1bar CO 2 and enables adjustable ether contents (TOF 75-956 h À1 ,8 0-120 8 8C, 0.025 mol %, 5-33 %P CHO links in PCHC);t his catalyst even retained good activity at 0.5 bar CO 2 and can switch between CHO ROPa nd CO 2 /CHO ROCOP when changing the reaction atmosphere from CO 2 to N 2 and vice versa. [57] Thep olycarbonates prepared by CO 2 /epoxide ROCOP are usually the kinetic reaction products,w hich provides an opportunity to chemically recycle them to either cyclic carbonates or the parent monomers. [3] Lu and co-workers reported ad i-Cr III catalyst for N-heterocyclic epoxide/CO 2 ROCOP which showed > 99 %p olymer selectivity at 60 8 8C ( Figure 11). [63] Nonetheless,a t1 00 8 8C, an ear quantitative depolymerisation occurred, re-forming the epoxide and CO 2 . Depolymerisation of the purified polymer back into monomers also occurred in the bulk phase at higher temperature (260-300 8 8C) without any catalyst. This result illustrates future potential in circular polymerisation/depolymerisation process,a lthough it should be emphasised that such al owtemperature depolymerisation could be problematic for polymer processing and may need optimization. Fully bioderived poly(limonene carbonate), prepared by CO 2 /limonene oxide (citrus fruit peel) ROCOP,w as also depolymerized to limonene oxide and CO 2 using either aZn II 2 complex or organic bases. [64,65] This depolymerisation is both monomer-a nd catalyst-dependent,s ince reports of PCHC depolymerisation indicate selective c5c formation, [57,[66][67][68] although, trans-c5c can undergo ring-opening polymerization to form polycarbonates. [69,70] Accordingly,C oates and co-workers reported an eat proof of chemical recycling using isotactic PCHC,s ynthesized using an enantioselective Zn II -bis(diketimide) catalyst. Va cuum thermolysis at 250 8 8Cs electively depolymerized it into trans-c5c in 95 %y ield;t he cyclic carbonate was subsequently efficiently repolymerized. [68] 3. Recent Trends in Anhydride/Epoxide ROCOP Another classic ring-opening copolymerisation is that of cyclic anhydrides with epoxides to yield polyesters. [13] It enables access to many different polyester backbone and sidechain reactions.Incontrast to heterocycle ROP, the ring strain of epoxides/anhydrides is less impacted by substituents and, thus,the polymerisation remains thermodynamically feasible using substituted/functionalized monomers. [71] It is also an excellent means to increase backbone "rigidity" through the incorporation of aromatic or strained heterocyclic units. Many epoxides and anhydrides are already large-scale chemical products and this may help accelerate the implementation of this polymerisation method. There have already been some comprehensive reviews on epoxide/anhydride ROCOP;h ere only recent developments in catalysis will be described, with af ocus on findings most relevant to other monomer combinations. [8,9] Here,m ost catalysts are benchmarked by performances using phthalic anhydride(PA)/CHO, but so far this field lacks common standards and the multitude of other monomers and reaction conditions complicate comparisons of the catalysts ( Figure 12).
Most catalysts active for CO 2 /epoxide are also active for anhydride/epoxide ROCOP,b ut the reverse isntn ecessarily true.F or example,L ewis base catalysts only form c5c with CO 2 /epoxides,b ut catalyse anhydride/epoxide ROCOP at Figure 10. Co II Mg II synergic heterodinuclear catalyst for CO 2 /CHO ROCOP compared with the Mg II Mg II and Co II Co II variants sheds light on the molecular basis for synergy. [54] Figure 11. Thermally controlled reversible CO 2 /epoxide polymerisation and depolymerisation. Copyright2 017 Wiley. [63] appropriate temperatures. [76,77] This reactivity difference stems from the side reactions:a lkoxide-terminated polymer chain ends back-bite into adjunct carbonate links to form c5c, but such ap athway has ah igher barrier in the case of anhydride/epoxide ROCOP.N evertheless,i nt he latter polymerisation, alkoxide chain ends can undergo transesterification, thereby broadening the molar mass distributions.
Tolman, Coates,a nd co-workers reported an excellent ROCOP mechanistic investigation using Al III -salen/PPNCl bicomponent catalysts ( Figure 13). [78] Ab is(carboxylate) aluminate resting state was proposed in the initial stages of the catalysis,even when alarge excess of epoxide was present. Therate-determining step was proposed as epoxide insertion into the aluminium-carboxylate intermediate to produce am ono(alkoxide)-mono(carboxylate) aluminate intermediate.R apid insertion of an anhydride monomer into this intermediate regenerated the bis(carboxylate) aluminate resting state.T he Al III -salen/PPNCl catalyst forms an ion pair and should be treated as such in any kinetic analyses.As the polymerisation progressed and the anhydride concentration became depleted, the bis(alkoxide) aluminate intermediate accumulated and undesired side reactions,s uch as transesterification, became feasible. Inspired by the mechanism, Coates and co-workers reported highly active aminocyclopropenium chloride tethered Al III -salen catalysts which maintained high activities at low catalyst loading (PO/norbornene anhydride,T OF 80 h À1 ,6 0 8 8C, 0.005 mol %, vs.T OF 10 h À1 for ab icomponent catalyst). [79] These properly designed tethered catalysts showed much less transesterification, epimerisation, and chain-end coupling reactions than bicomponent analogues.T he performances were rationalised by control over the metalate equilibria avoiding formation of free alkoxide chains.T he tethered catalyst was also tolerant of large quantities of chain transfer agent, thereby allowing control of the molar mass. [81] In bicomponent systems,c hain transfer agents reduce the propagating chain nucleophilicity,t hrough hydrogen bonding,a nd suppress metalate formation through competitive coordination to the metal centre.The tethered system intrinsically favours metalate formation and hence counterbalances deleterious influences of chain transfer agents and allows access to branched and star polymers.
Coates and co-workers also developed Al III -salen catalysts with electron-withdrawing para-fluoro substituents,w hich significantly reduced transesterification. [80] It was proposed that the substituents stabilize the aluminate and prevent dissociation of the alkoxide chain end from the catalyst, ah ypothesis reminiscent of those rationalizing the enhanced performances of tethered catalysts systems in CO 2 /epoxide ROCOP.
Ammonium salts R 4 NX (X = Cl, Br, I, N 3 ,O Ac) with ab is-hydrogen bond donor, such as I 2 or BEt 3 ,w ere also active catalysts (Figure 17). [93][94][95][96] It was proposed that oxetane activation occurred by borane coordination, hydrogen bond-ing, or halogen bonding.Some organocatalysts were effective when using unpurified monomers,a lthough the resulting molar masses of the polycarbonates were very low (< 2kgmol À1 ), likely due to contamination by ap rotic compound. Although readily available,a ll these organocatalysts required high loadings of 0.5-3 mol %, show only modest activities (TOF 5h À1 , ! 90 8 8C), and produced variable c6c/ PTC product ratios depending on the reaction conditions.
Improving catalyst performances as well as tackling monomer purity issues to drive up molar mass values (both of which are lagging behind what has been achieved with CO 2 / epoxide) will be essential to yield useful PTC which, when prepared by other means,s hows promise in biomedical applications. [97,98] PTMC (T g ca.À20 8 8C) has been used to form matrices for cell growth, with ap articular focus on the regeneration of bone,c artilage,n erve,a nd/or blood vessels. [99][100][101][102] PTC undergoes rapid hydrolysis under biologically relevant conditions,b ut unlike aliphatic polyesters does not generate acidic decomposition products and thus can reduce inflammation side effects. [103] Such properties are important for any biomedical implants targeted to fully degrade after healing. [104] As acomponent in block polymers,italso allows the controlled release of anti-cancer drugs,p roteins,a nd gene-therapeutics. [105][106][107]
Polymers containing sulfur or selenium links can be oxidative responsive materials.F or example,b lock copolymers containing ac halcogen-containing hydrophobic block with ah ydrophilic block self-assembled into micelles when dispersed in water. Upon oxidation with, for example,H 2 O 2 , the chalcogen centres become hydrophilic, thereby causing the micelles to disassemble,a sd emonstrated with ab lock polymer prepared by SBL/epoxide ROCOP. [131,133,134] This property was explored for drug delivery in arecent report of ap olymer containing selenoether and PEG blocks.T hese micelles were loaded with doxorubicin and ap hoto-oxidant, with the anti-cancer drug released upon irradiation with IR light. [135]
Thec opolymerisation of either sBL or fBL resulted in volume expansion, aconsequence of the double ring opening of the fused/spiro cycle,w hich may be useful for polymer resins. [140] Whereas epoxide ROPresults in volume shrinkage, isopropyl-fBL/PGE ROCOP had the same volume before and after polymerisation. In resin applications,s hrinkage is undesirable,e specially for adhesive or filler uses,a nd may lead to sub-optimal interfaces and performances.E ndo and co-workers also reported isoprene-substituted bBL which, after ROCOP with PGE, was functionalized through thiolene reactions or radically cross-linked to form networks. [142] Thel atter networks showed increased rigidity and the expected improvement in thermal stability (which increased by 50 8 8Ct ogive T d = 326 8 8C).

ROCOP of COS with Epoxides and Oxetane
Carbonyl sulfide (COS), the monosulfur analogue of CO 2 , is both an aturally occurring gas (e.g.r eleased by marine plants or volcanic eruptions) and an anthropogenic environmental pollutant emitted by burning fossil fuels. [145,146] It is amajor source of acid rain as it can be oxidized to SO 2 in the troposphere and, furthermore,d amages the ozonosphere. Therefore,i ts use as am onomer could redress the environmental impact and valorise an industrial waste. [147,148] In 2013 Zhang and co-workers successfully achieved both chemo-and regioselective COS/PO ROCOP using aC r IIIsalen/PPNCl catalyst system to produce monothio-PPC (TOF 288-332 h À1 ,2 5 8 8C, 0.1 mol %, M n = 21.9-25.3 kg mol À1 ). [149] Theselectivity for monothiocarbonate (-(S-)C(=O)-O-) linkages was attributed to the preferential coordination of sulfur (rather than oxygen) to Cr III .The increased nucleophilicity of the Cr III -thiolate intermediate (compared with Cr III -alkoxides) was attributed to the higher activities compared to those for CO 2 /PO ROCOP.T he thiocarbonate linkage is asymmetric, so copolymerisation with monosubstituted epoxides may form four regioisomeric linkages:HT, TH, TT,and HH. The notation describes whether the CH 2 (T) or CHR (H) group sits adjacent to the respective sides of the monothiocarbonate links ( Figure 21);t he Cr III -salen catalyst system showed remarkably high selectivity for TH linkages (> 98 %). The polymerisation conditions needed to be quite finely balanced, since am oderate increase in the temperature (60 8 8C) or adventitious moisture formed cyclic carbonate,d ithiocarbonate (-SC( = O)S-), and carbonate (-OC-(=O)O-) linkages by O/S scrambling. [150] TheC r IIIsalen/PPNCl catalysts also  . [151][152][153] Theg lass transition temperatures of the poly(monothiocarbonates) are similar to those of the polycarbonate analogues and can be easily controlled over aw ide temperature range by changing the epoxide (T g = 3-115 8 8C). Many polymers are optically transparent, with high refractive indices,f or example,R I = 1.63 for COS/PO or 1.70 for COS/CHO. [147] To avoid chromatic aberration in optical applications,t he RI should remain constant as the refracted light wavelength changes,aproperty expressed by the Abbe number V d (i.e.higher V d values are better for uses as lenses, prisms,o rw aveguides). [154] In COS/epoxide ROCOP, V d was controlled, and could be increased, by forming random polymers through terpolymerisation. Fore xample,C OS/PO/ CHO ROCOP leads to tuneable V d values (32.1-43.1), high RI values (1.52-1.56), and T g = 44-93 8 8C, depending on the CHO/PO feed ratios. [152] CHO/CO 2 /COS ROCOP,u sing an [Al III salen] 2 /PPNCl catalyst, gave polymers with the highest V d value of 48.6 for randomized equimolar carbonate/ thiocarbonate links while maintaining high glass transition and decomposition temperatures (T g = 111 8 8Ca nd T d = 260 8 8C). [155] As was also observed for CO 2 /epoxide ROCOP,tethered catalysts showed the highest activity and selectivity as well as excellent performance at the lowest catalyst loadings.F or example,aDBU-tethered Cr III -salen catalyst operates at high polymerisation temperatures,t hereby resulting in very high activity without compromising the O/S scrambling or polymer selectivity (TOF 4670-271 000 h À1 ,2 5-80 8 8C, 0.005-0.00005 mol %, 27.1-220.0 kg mol À1 ). [156] In direct contrast to CO 2 /PO ROCOP,w here Co III catalysts were usually more active than Cr III systems, the reverse was observed for COS/PO ROCOP.T he Cr III -salen catalyst was also active for other COS/monosubstituted epoxide ROCOP reactions,a nd always showed high activity (TOF > 20 000 h À1 ). Tw o slower copolymerisations-COS/CHO (260 h À1 )a nd COS/CPO ROCOP (1360 h À1 )-were accelerated when 1mol %P Ow as added (TOF 15 800 h À1 ). Theh eterogeneous Zn/ Co III -DMC catalysts were also active in COS/CHO ROCOP (190 gg À1 h À1 , 110 8 8C, M n = 6.5-25.0 kg mol À1 ), forming colourless polymers (c.f.t he Cr II -salen/PPNCl catalysts yielded pale yellow polymers even after repeated washing), although with 10 % O/S scrambled linkages. [157] Recently semicrystalline poly(monothiocarbonates), derived from COS/epoxide ROCOP,were made by polymerizing enantiopure ECH with COS using aDBU-tethered Cr IIIsalen catalyst (TOF 19 h À1 , À25 8 8C, 0.1 mol %, M n = 3.1 kg mol À1 , T g = 16 8 8C, T m = 97 8 8C). [159] Thep olymerisation suffered from termination reactions,w ith the molar mass values being very low,a nd these occurred by nucleophilic substitution reactions between the growing polymer chain and the chloride substituent of ECH ( Figure 22). Nonetheless, these epoxides underwent asecond COS/epoxide ROCOP to yield semi-crystalline graft polymers. [158] In atwo-step process, the length of the substituent on the epoxide-terminated "macromonomer" was initially determined by the reaction temperature (À20 to 0 8 8C). In the second step,t he temperature was increased to 25 8 8Ca nd the macromonomer underwent ROCOP to form graft polymers (M n = 32.9-37.8 kg mol À1 ,w ith 1.5-1.8 kg mol À1 branches). Both ROCOP processes were highly controlled and about 90 % ECH was converted before formation of the graft polymer. Naturally,the graft polymer showed different thermal properties to the starting macromonomers (T g = 11 8 8C, T m = 113 8 8C, T d = 262 8 8C). As graft polymers could be readily formed by ROCOP,either with multi-functional chain transfer agents or through temperature switching,t his might be ap romising future direction for these polymers. [23] Generally,g raft polymers show different rheology and viscosity compared to their linear components because of different chain entangle- Figure 21. COS/epoxide ROCOP illustratingd ifferent chemo-and regioselectivities. [149] . Figure 22. Semicrystalline graft polymers produced by two-stage COS/ECH ROCOP. [158] Angewandte Chemie Reviews Angew ment. Therefore,branched polymer solutions are often much less viscous than their linear counterparts,w hich may facilitate processing and applications,f or example,i nt he administration of liquid drug formulations which are thus easier to inject by syringe. [160,161] Branched polymers can host ag uest molecule through non-covalent encapsulation within the cavities between the chains-a relevant feature for the delivery and controlled release of pharmaceuticals. [162] Isotactic COS/PO and COS/PGE copolymers,p repared from enantiopure epoxides,w ere amorphous,b ut the COS/ EO polymer was semicrystalline (T m = 128 8 8C, T c = 66 8 8C; note that the all-oxygen variant poly(ethylene carbonate) derived from CO 2 /EO ROCOP is amorphous (T g = 0-10 8 8C). [149,152,158] Thec atalyst system was aD BU-tethered Cr III -salen complex and it showed very high activity (TOF 84 900 h À1 )toproduce ahigh molecular weight polymer (M n = 193 kg mol À1 ). An ABAt riblock polymer (COS/EO-b-COS/ PO-b-COS/EO), which features as emicrystalline-soft-semicrystalline combination (30 %C OS/EO units,7 0% COS/PO units), with am oderate molecular weight (M n = 13 kg mol À1 ) showed as tress at breaking of 11.2 AE 0.1 MPa and an elongation at breakage of 575 AE 52 %. [163] Thet hermoplastic elastomer showed an elastic recovery of about 90 %after five 300 %s train cycles;a lthough the performance cannot match existing thermoplastics,f or example,s tyrene/butadiene SBS (28 MPa stress and 800 %e longation at break) or polyurethanes PU (25-75 MPa stress and 500 %e longation at breaking point), it serves as proof of potential ( Figure 23). [164] Lu and co-workers employed an asymmetric catalyst to make semi-crystalline poly(monothiocarbonates) from achiral meso-epoxides and COS. [165] In the case of COS/CHO ROCOP,[ Cr III salen] 2 yielded atactic polymer,b ut the Co III analogue produced highly isotactic polymer (P m = 90 %). The extent of the isotacticity of course controlled the maximum melting temperature,w ith values up to 201 8 8Co bserved at 99 %i sotacticity (M n = 29.5 kg mol À1 ). Thes ame stategy was also successful for the stereoselective ROCOP of cyclopentene oxide or 3,4-epoxytetrahydrofuran, which both formed highly isotactic polymers (T m = 141 and 232 8 8C, respectively).

ROCOP of CS 2 with
Epoxides, Oxetane, and Thiirane CS 2 /PO ROCOP was first reported in the 1970s by Adachi et al.,w ho used am ixture of Et 2 Zn and HMPA( TOF < 1h À1 ,2 58 8C, M n = 0.6 kg mol À1 ). [176] Tw o key findings resulted:a )O/  Ss crambling formed -S-C( = S)-O-linkages as well as other linkages ( Figure 25);b )the CS 2 /epoxide system dramatically influenced the catalytic activity and selectivity.T his latter observation is different from CO 2 /epoxide ROCOP,w here the catalytic performance is commonly independent of CO 2 pressure.
Heterogeneous DMC catalysts for CS 2 /PO ROCOP yielded polymers with low molecular weights and with moderate/good activities (52-182 gg À1 h, M n = 1.2-5.4 kg mol À1 ). [177] Ther esulting polymers were oxygenenriched (ca. 35 %t heoretical max. S) and exhibited nearly all the possible permutations of linkages.Moreover,carbonyl sulfide (COS) was detected in increasing quantities as the reaction progressed. This was attributed to COS being more easily,orperhaps rapidly,incorporated, thus rationalizing the observation that -S-C(=O)-O-linkages were the most prevalent.
Clearly,c ontrolling the O/S scrambling side reactions is as ignificant challenge in CS 2 /epoxide ROCOP.U nderstanding pathways that form particular linkages remains more hypothetical than proven;n onetheless,i fi tw ere possible to control these processes in the future it might be feasible to produce polymers featuring (ABC) n or (ABCD) n monomer sequences instead of the expected, and more common, (AB) n .
In 2016 Werner and co-workers realized an (ABAC) n copolymer, with 92 %s equence selectivity,f rom CS 2 with PO or butylene oxide and LiO t Bu as the catalyst (TOF 50 h À1 , 25 8 8C, 0.125 mol %, M n = 132 kg mol À1 ). [182] Thec atalyst leads to chains with as omewhat unusual HH-TT selectivity and at entative mechanism was proposed ( Figure 26). Accordingly,t he propagating alkoxide may attack an adjacent -O-(C = S)-S-linkage to afford a-O-(C = S)-O-linkage with concomitant formation of at hiolate chain end. Thes ame lithium alkoxide catalyst also produced an isotactic polymer from CS 2 /R-PO ROCOP which has ahigher T g value than the atactic form (T g = 30 8 8Cf or isotactic vs.1 3 8 8Cf or atactic). Organocatalysts composed of Et 3 B/lewis base were also effective for CS 2 /PGE ROCOP and afforded only trithio-or monothiocarbonate linkages,a lthough with worse overall performance than metal systems. [183] CS 2 /EO ROCOP using bicomponent Et 3 Bo rC r III -salen catalysts revealed that the extent of the O/S scrambling changed the material properties from completely amorphous (T g = À18.6 to À35.1 8 8C) to highly crystalline (T g %À34 8 8C, T m = 118-211 8 8C; Figure 27).
Whereas sulfur-rich segments tend to be crystalline,t he all-oxygen carbonate linkages form amorphous regions. Another interesting feature of these copolymers is their ability to be degraded by oxidants.T he immersion of solid polymer in 30 %H 2 O 2 for 12 hr esulted in degradation to oligomers (M w = 1.8 kg mol À1 )w ith formation of sulfones (R 2 SO 2 )a nd sulfonic acids (RSO 3 H). [184] Figure 25. CS 2 /epoxide ROCOP and 13 C{ 1 H} NMR spectrum illustrating the polymer functionalities accessible when O/S scrambling occurs. Figure 26. CS 2 /epoxide ROCOP,c atalysedb yLiO t Bu, to produce (ABAC) n copolymer;R= Me, Et. [182] Figure 27. XRD data for CS 2 /EO copolymers from bicomponent catalysts show different degrees of crystallinity. [184] Copyright 2020 Wiley.

Reviews
Early investigations of CS 2 /ES and PS ROCOP used highly toxic CdEt 2 or Hg(SBu) 2 catalysts-or metal-phenolate catalysts in the case of CO 2 /PS ROCO-and formed large quantities of thioether as well as heterocarbonate links. [185][186][187] Nozaki and co-workers reported CS 2 /PS ROCOP using aCr III -salen/PPNCl catalyst, which produced highly alternating trithio-PPC with high polymer selectivity (92 %) and good activity (TOF 76 h À1 ,2 5 8 8C, 0.2 mol %, M n = 44.6 kg mol À1 , T g = 25 8 8C, T d > 200 8 8C; Figure 28). [188] Thep oly(trithiocarbonate) was only sparingly soluble in common organic solvents but was highly soluble in CS 2 ,t hereby highlighting future processing challenges for highly heteroatom-rich polymers.T he polymers showed high refractive indices (RI = 1.78 for CS 2 /PS,1 .73 for CHS/PS;i nc omparison, the RI values of PPC is 1.46 and PCHC is 1.48). In general, high RI polymers are investigated for their optoelectronic applications,s uch as for lenses in image sensors,o ptical layers in LCD displays,e ncapsulants for LEDs,a nd anti-reflection coatings. [154,189] Compared to alternative inorganic compounds,the ROCOP polymers may benefit from mechanical flexibility,i mpact strength, processability by moulding or casting,h ave low molecular weights,a nd potentially have lower costs.A lthough S-containing ROCOP polymers have not been explored for such applications,the broad monomer scope and potential for terpolymerisation may allow future optimisation of the properties.
Both CS 2 /PS and CS 2 /CHS copolymers exhibited antimicrobial activity against Escherichia coli and Staphylococcus aureus. [190] Bacterial cultures in contact with polymer films were assessed for their cell viability by counting surviving colonies.A lthough poly(cyclohexene trithiocarbonate) was more effective against E. coli (20 %c ell viability for CS 2 /PS and > 10 %for CS 2 /CHS after 24 hcontact time), the biocidal performance was reversed for S. aureus (25 %cell viability for CS 2 /PS and 50 %f or CS 2 /CHS after 24 hcontact time).
Polymerisation control was good, and block polymers were prepared by the addition of asecond thioanhydride after consumption of the first. Thec opolymer showed ah igh refractive (RI = 1.79) and moderate melting temperature (T m = 80 8 8C), with the latter value increasing to 90 8 8Cwhen R-PS was used. Theonset of thermal decomposition occurred at 230-300 8 8C( T g = À22-60 8 8C) depending on the thiirane employed, which suggests the materials have ar easonable processing window.

Angewandte Chemie
Reviews on sulfur reactions that might be relevant to biological compatibility and biodegradation. [194]

ROCOP of S 8 with Thiiranes
Ther ing-opening polymerisation of elemental sulfur (S 8 ) is thermodynamically disfavoured at temperatures below 159 8 8Cb ut occurs readily at higher temperatures. [195] This unusual phenomenon arises because S 8 ROPi se ntropically favoured, that is, DS(polymerisation) > 0. [196] Elemental sulfur is aw aste product of petrochemical refining,w ith 70 Mt produced annually,m ost of it by hydrodesulfurisation. [197] Although as ignificant portion is used to make sulfuric acid, rubber, and fertilizer,excess sulfur accumulates in large overground storage facilities.
Analysis of the initial rates demonstrated that the sulfide chain end attacked S 8 about 10 times faster than thiirane.The -S 9 À intermediate attacked thiirane about 100 times faster than it formed S 8 .C uriously,t he anionic polysulfide chain ends became less reactive as the preceding S n group gets longer, thus suggesting some negative charge delocalisation along the chain. Similar to other ROCOPs,s mall cyclic byproducts were formed, including cyclic tri-, tetra-, and pentasulfides.T hese cyclic compounds reached am aximum concentration and then decreased over time,t hus suggesting they undergo ring-opening polymerization;t his was later confirmed by homo-and copolymerisation with S 8 . [201] Sulfur/ heterocycle ROCOP reactions yielded polymers which are amorphous elastomers (M n = 10-100 kg mol À1 ). In contrast to polysulfides S n ,t he polymers were stable with respect to depolymerisation as well as to extrusion of sulfur under ambient conditions.F ilms cast from polymer solutions retained transparencya fter storage for 4years.P oly(styrenealt-sulfur), obtained from S 8 /styrene sulfide ROCOP,showed that increasing the sulfur content (although not quantified) decreased the T g value from 58 to 43 8 8C, which matched previous reports that increasing the wt %o f sulfur reduced the brittleness. [203] It is emphasised that sulfur-sulfur bonds are often dynamic,aproperty exploited in autonomously self-healing polyurethane elastomers,w here macroscopic damage was healed through dynamic disulfide bonds. [204] By simply bringing two ends of the cut specimen bar into contact, the mechanical properties were nearly completely restored after 24 h. In the future,t hese ROCOP polymers could be explored as self-healing materials.

ROCOP of SO 2 with Epoxides
Sometimes ROCOP occurs spontaneously because the two monomers (A and B) first form an activated monomer adduct (A-B), which then forms chains by step growth or other mechanisms.
Such an activated monomer mechanism was reported for SO 2 /epoxide ROCOP,w hich formed low molar mass polymers enriched with ether linkages (Figure 32). [205][206][207][208][209] Higher molar masses,s ulfate incorporation, and reaction rates were achieved using Lewis base initiators such as N-heterocycles, phosphines,o rs alts (TOF 20-35 h À1 , 50 8 8C, 0.1-1 mol % pyridine, M n = 11.9-14.2 kg mol À1 ). Initiation occurred from zwitterions (LB + CR 2 CR 2 OSO 2 À )and the activity was dependent on the ionization of the initiator.P ropagation occurs when SO 2 activates the epoxide and, in general, the rates depend on the epoxide,t he SO 2 ,a nd Lewis base.C yclic and linear polymers form, which limits the molar masses obtained (e.g. SO 2 /EO ROCOP = 9.2-14.2 kg mol À1 ). Using poly(vinylpyridine) as an initiator allows separation of the linear and cyclic polymers which are bound to the macro-initiator. Thermal decomposition of the SO 2 /EO copolymer, at relatively low temperature (T d = 216 8 8C), led to mixtures of cyclic five-membered sulfites and EO,w hich were recovered in about 44 %y ield. Thed epolymerisation was catalysed by

ROCOP of RNCO with Epoxides and RNCS with Thiiranes
Early studies demonstrated that slow alternating ArNCO/ EO ROCOP was possible when employing aAlEt 3 /H 2 O(2:1) catalyst system (TOF 0.3 h À1 ,2 58 8Cf or PhNCO). [215,216] The addition of the isocyanate to the epoxide occurred with retention of the C = Nb ond to form an acetal linkage or with retention of the C=Ob ond to form au rethane linkage ( Figure 34);t he semi-crystalline polymer contained two thirds acetal linkages (M n = 1-2.1 kg mol À1 , T m = 80-83 8 8C). In comparison, an isomeric polymer synthesized by ROPand that only contains urethane linkages showed a T m = 192 8 8C. These findings emphasize the importance of linkage connectivity in moderating material properties.I ts hould be noted that alternating polyurethanes,s uch as those formed through ROCOP,a re totally different to the polyurethanes currently produced through the condensation of polyols and di-isocyanates. [7] These commercial products contain am uch lower wt %o furethane linkages and combine hard and soft domains which are integral to their properties.
Faster and more selective ArNCO/CHO ROCOP (TOF > 1000 h À1 ,808 8C, 0.2 mol %, M n = 3-8 kg mol À1 )was achieved using aM g II 2 catalyst [LMg 2 OAc 2 ], but curiously the di-Zn II and di-Co III derivatives as well as the Co III -a nd Cr III -salen/ PPNCl catalysts were completely inactive. [ [218] Using monofunctional chloride initiators yielded polyurethanes showing monomodal molar mass distributions, whilst phosphazene and diol catalysts yielded the more useful telechelic polyurethanes.T he TsNCO/PO copolymer was amorphous with T g = 107 8 8C( M n = 20 kg mol À1 , T d = 242 8 8C), which is significantly higher than that of the related polycarbonate PPC (T g = 22-40 8 8C, T d % 242 8 8C). By changing the epoxide,i tw as feasible to control T g in the resulting polyurethanes to give values as low as À24 8 8C; it was also noted that TsNCO/EO yields asemi-crystalline polyurethane, with In related study, Oct 4 NBr/(iBu) 3 Al was investigated for ArNCO/butylene oxide ROCOP,b ut gave poor polymer and linkage selectivity. [219] Using n BuLi as the initiator resulted in successful RNCS/ ES ROCOP to produce semi-crystalline poly(imino dithioacetal), [-CH 2 CH 2 -S-C(=NR)-S-] n (T m = 63-128 8 8Cdepending on R; M n = 25-60 kg mol À1 ). [220] Polymerisation rates were increased when using coordinating solvents,f or example, THF or (Me 2 N) 3 P = O, which enhance the nucleophilicity of the dithiocarbamate chain end through its coordination to lithium. Thet erpolymerisation of different RNCS species with ES followed the reactivity ratio trends,w ith aryl-NCS being more reactive than alkyl-NCS.F or example,P hNCS/ EtNCS/ES ROCOP selectively formed the PhNCS/ES block Figure 33. MALDI-TOF MS spectrum of aSO 2 /CHO copolymer showing chains initiated from trace amounts of H 2 O. [210] Copyright2 020 American Chemical Society. first followed by the EtNCS/ES block. When ES was used in excess it was also consumed by ROP, thereby forming afinal poly(thioether) block, together with slow degradation of the polymer.T he RS-(C = NPh)-SR repeat units can be hydrolysed upon contact with dilute aqueous acids,t hereby resulting in the release of aniline (PhNH 2 )a nd formation of semicrystalline polymers with RS-(C=O)-SR linkages which are much less soluble than the starting polymer.W ua nd coworkers reported strictly alternating RNCS/thiirane ROCOP, by using PPNCl initiators,a nd achieved good to excellent activity and molar masses (TOF 21-1162 h À1 ,0 .4-0.1 mol %, 25-100 8 8C, M n = 19.9-142 kg mol À1 ). [221] Thed ifferent monomer substitution patterns highlight the wide range of structures accessible with this ROCOP method ( Figure 35).

ROCOP Involving Aziridines
Aziridines are more nucleophilic than epoxides or thiiranes by virtue of the lone pair of electrons on the Na tom. Hence,t hey form stable carbamate adducts with carbon dioxide,e ffectively the reverse of the reactivity with SO 2 , where adduct formation resulted from its increased electrophilicity.F or example,e thyl aziridine (EI) reacted with CO 2 at À27 8 8Ctoform the salt [EICO 2 ] À [EIH] + .Upon heating this salt in mixtures of EI and CO 2 ,the anion [EICO 2 ] À attacked and ring-opened the activated aziridine cation [EIH] + to effect copolymerisation by an activated monomer mechanism ( Figure 36). [222,223] Twoc ompetitive polymerisation pathways were observed in parallel:1 )F ormation of polyamine linkages by cationic aziridine ROPa nd 2) branching from these amine links.

Angewandte Chemie
Reviews it, presumably as ac onsequence of intermolecular hydrogen bonding ( Figure 37). Using dimethylacetamide as aco-solvent prevented precipitation and homogenized the mixture, thereby allowing the isolation of polymers with higher molar masses (M n = 210 kg mol À1 ,7 4% urethane linkages).
Thep olymers contain hydrophilic polyamine and hydrophobic polyurethane sequences in various proportions and so, depending on the reaction conditions (CO 2 pressure,solvent, etc), they show different lower critical solution temperatures (LCST) in water. TheL CST arises from an unfavourable entropy of mixing between the polymer and solvent and is observed for other polymers,w ith ac ommon example being poly(N-isopropylacrylamide). [230] In the case of these CO 2 / aziridine copolymers,t he amine linkages are susceptible to protonation/deprotonation and thus the solution pH also controls the LCST: under acidic conditions there is no LCST and its value decreases as the pH increases. [229] Ikeda and co-workers reported that water-catalysed CS 2 /N-cyanoethyl-EI ROCOP formed ap oly(dithiocarbamate) which was semi-crystalline (T m = 155 8 8C) and poorly soluble in all solvents,e xcept for DMSO ( Figure 38). [231] The rate law showed dependen-cies of:[ N-EtCN-EI] 2 [CS 2 ][H 2 O] and was interpreted by ar ate-determining step involving attack by an N-C( = S)-S À group from an activated monomer complex on ap rotonated aziridine (protonation from water). TheC S 2 /aziridine copolymer showed high alternation, irrespective of the starting quantities of each monomer,thus suggesting that the aziridine ROPw as significantly less favourable compared to other CO 2 /aziridine ROCOPs.
COS/aziridine ROCOP is also spontaneous through an activated monomer mechanism. [234] Using COS pressures of 10 to 20 MPa results in the formation of polymers with molar masses up to 15 kg mol À1 within 2hours at room temperature. [232,233] Theh ighly alternating polymers were macrocyclic, as determined by MALDI-TOFm ass spectrometry, and were cleanly depolymerized at 200 8 8Ci nto the fivemembered cyclic thiourethane.T hese polymers were shown to effectively absorb heavy metal salts,s uch as HgCl 2 and PbCl 2 ,from aqueous solutions.The absorbed metal salts were isolated after thermal depolymerisation and removal of the small molecule cyclic compounds by distillation ( Figure 39). Whereas the COS/PI copolymer was amorphous (T g = 90 8 8C), the COS/(N-ethyl-EI) (T m = 170 8 8C) and N-butyl-Az (T m = 137 8 8C) copolymers are semi-crystalline.There is also atentative precedence for spontaneous COSe/aziridine copolymerisation. [235] Poly(ester-amide)s,t he formal products of anhydride/ aziridine ROCOP,combine the biodegradability of polyesters with the thermal and mechanical properties of polyamidesthese desirable features have resulted in applications ranging from drug delivery systems to hydrogels,composite matrices, and tissue engineering scaffolds. [236] In some cases,anhydride/ aziridine ROCOP was spontaneous,but was characterized by poor control over the molar mass and amine linkages. [237,238] Quantitative monomer alternation was achieved with Lewis base/alcohol organocatalysts.C yclic polymers were isolated in some cases when mono-/bicyclic anhydrides and N-benzylsubstituted aziridines were applied (TOF 1-10 h À1 ,708 8C, 0.2-1mol %, M n = 4.4-34.1 kg mol À1 , T g = 41-126 8 8C, T d = 258-311 8 8C). [239] As with cyclic polymers formed by macrocyclisation during termination, applying higher BnOH loadings resulted in more linear chains.

Conclusions and Outlook
Heterocycle/heteroallene ROCOP allows efficient and selective formation of many completely new and sophisticated polymer microstructures from comparatively simple monomers.O ptimizing the catalysis should allow both the tailoring of the polymerization kinetics and delivery of useful materials for future applications to be prioritized. Although great improvements in carbon dioxide/epoxide ROCOP catalysis have been achieved, in many other cases the catalysis is really at an early stage.K ey targets are increased reaction rates and control over tacticity,molar mass,linkage,and chain end reactions.O ne strategy is to seek inspiration from the demonstrated successful approaches in CO 2 /epoxide ROCOP catalysis.These include developing co-catalyst-tethered metal complexes/organocatalystsbyexploiting heterodinuclear synergy and applying chiral or non-initiating organometallic catalysts to properly control the stereo-or end-group chemistry of the chain. There are,h owever, some monomer combinations for which optimized carbon dioxide ROCOP catalysts are unsuccessful and, in these cases,t he field needs ab etter understanding of the kinetics and mechanisms so as to rationally improve performances.T he polymerization catalysis community should feel optimistic in these endeavours,s ince the range and scope of ROCOP catalysts is still very narrow,w ith many Lewis acid and labile metal/organocatalysts remaining to be explored. Another priority is to develop tolerant catalysts and processes using impure monomers or mixtures-such systems would be highly attractive for large-scale deployment but could also accelerate uptake by the broader polymer chemistry community.
In this Review,w eh ave tried to highlight the immense potential for many under-explored monomer combinations to deliver functionalised polymers and materials.T he polymer chemistry and physics of these materials is at av ery early stage and the field will benefit from the attention of those with expertise in processing, properties,and applications.One area worth immediate investigation is to use multi-functional chain-transfer agents to target new polymer architectures and topologies,f or example explorations of star and brush polymers.A nother is to exploit recently discovered switchable polymerization catalysis to deliver block-sequenceselective copolymers. [241] Thep recise monomer placement within polymer chains afforded by switch catalysis has already shown promise in the delivery of ductile plastics,a dhesives, and thermoplastic elastomers. [60,65,242,243] In the future,i ts application with heteroatom-functionalized polymers could be used to broaden into sectors including engineering plastics, fibre-compatible resins,s oft robotics,i onic conductors,a nd medical materials.
Ring-opening copolymerization processes and polymers may be of interest to tackle UN Sustainable Development Goals (SDGs). Overall, polymer sustainability can only be assessed through life-cycle assessments and are applicationspecific, but there are some features of these polymers that meet criteria for sustainable polymers.F or example,m any monomers are existing industrial wastes and others could be bio-derived. It is recommended that catalysis and polymer chemistry research should target such non-petrochemical materials for development. TheR OCOP process has high atom economy and may be suitable for retro-fit into existing manufacturing and processing infrastructure.P olymer properties are well-matched with growth industries in renewable energy generation, increased use of biomaterials such as wood/paper, or in delivering self-repairing products.Interms of end-life options,s ome of these polymers show promising characteristics for circular chemical recycling.T hese heteroatom-containing backbones appear to facilitate depolymerisation to monomers or small cyclic molecules suitable for repolymerization under accessible conditions.N onetheless, such properties must be carefully balanced with processing and application performances.O ther heteroatom-containing polymer scaffolds are biodegradable and are already finding application in medical sectors,w hich indicates that the byproducts of degradation are not toxic. Taken as awhole,such features highlight the potential of this interesting class of polymers in helping to tackle the problems of todays materials.M uch more research is needed to improve their production, better understand their properties,a nd fully assess their life cycles. Figure 40. Anhydride/aziridine ROCOP data. Left:Insitu FTIR spectroscopic analysis of PA/N-Ts-EI ROCOP. [240] Right:DSC thermogram of the copolymer.C opyright 2020 Wiley.